The present invention relates to a pyrotechnic composition which, when burned, will produce a dense smoke particularly adapted for military purposes, such as for signaling or for camouflage.
Chemicals in the category of screening smokes are those which, when dispersed in air, produce a cloud of finely divided particles of solid, liquid, or both. These are used to shield tactical operations or disrupt the movements of the enemy. Outstanding examples of such materials are: fuel oil used in "artificial fog" generators, white phosphorus, sulfur trioxide, titanium tetrachloride, and so called zinc chloride smokes. Each of the above-listed smoke-generating compositions is characterized by certain advantages and disadvantages in military operations, depending upon the importance of such factors as mobility of the smoke producing apparatus, toxicity, logistical considerations, and the total obscuring power of the composition employed.
For military use, volatile hygroscopic chloride (HC) smokes are the most important, other than oil mixtures, which are utilized for large scale operations. The most widely used HC types of smokes are those resulting in the production of zinc chloride smokes.
The original mixture employed to produce a zinc chloride smoke was the Berger mixture, developed by the French Army during World War I. The original Berger mixture consisted of zinc dust and carbon tetrachloride with zinc oxide and diatomite. Upon ignition, a vigorous reaction takes place, resulting in the formation of zinc chloride, which is volatilized by the heat of the reaction and solidifies to form smoke. However, since this mixture employed a liquid organic chloride, it was difficult to transport and store. By the beginning of World War II, the United States Government had developed a mixture designated "HC smoke mixture" which contained zinc, a perchlorate as an oxidizing agent, hexachloroethane as the organic chloride compound, with a retarder, ammonium chloride. Subsequently, a mixture was found which was better in many ways than the original; it was a combination of hexachloroethane, aluminum and zinc oxide. This mixture required no stabilizer against moisture absorption, and changing the percentage of aluminum varied the burning time, as desired. However, these compositions are corrosive and will interfere with firing mechanisms, thereby materially limiting the storage life of the smoke-generating composition.
More recently developed pyrotechnic devices in use by the military use red phosphorus compositions, and a typical red phosphorus composition contains 51% phosphorus, 35% pyrolusite (MnO.sub.2), 8% magnesium, 3% ZnO and 3% binder. When this composition is ignited, the magnesium and MnO.sub.2 react as a thermite to generate heat and vaporize the phosphorus. The phosphorus vapor then burns in the surrounding atmosphere to produce a dense white smoke and a yellow flame. The phosphorus composition is typically long burning with an average rate of 2.42 .times. 10.sup.-2 cm/sec.
Another smoke composition containing red phosphorus is described in U.S. Pat. No. 3,607,472, which issued Sept. 21, 1971, to Bernard E. Douda. This patent describes a white smoke producing composition comprised of between 8 and 12 percent of magnesium, between 30 and 34 percent of magnesium dioxide, between 0 manganese 3 percent of zinc oxide, between 0 and 4 percent of lead dioxide, between 38 and 45 percent of red phosphorus and between 10 and 18 percent of a resinuous binder.
The phosphorus compositions presently being used by the military suffer from two basic problems. One problem is that one of the key ingredients, pyrolusite (MnO.sub.2), is not mined in the United States and thus is a foreign-supplied material. The pyrolusite which is presently used is an ore and has as much as 20 percent impurities.
A second and more severe problem with presently used phosphorus compositions is the production of hydrogen in the sealed pyrotechnic devices. This problem is caused by the reaction of magnesium with water trapped in the device during production for it is known that magnesium reacts with water according to the equation: EQU Mg + 2H.sub.2 O .fwdarw. Mg(OH).sub.2 + H.sub.2 ( 1)
the reaction rate is increased at higher temperatures and the generation of hydrogen in a sealed pyrotechnic system leads to severe pressure build-ups which can result in explosion and accidental ignition of the units. Such a reaction could produce catastrophic results, particularly aboard a ship where large quantities of munitions might be stored.